mack - 8:00 pm on May 25, 2010 (gmt 0)
WebmasterWorld is certainly big enough to operate its own ad network. My suggestion would be self serve, prepaid but manually approved before going live. I think text ads clearly marked as such would be less obtrusive than banners.
If you where to run banners I think Flash ads would be a bad choice. I suspect iPhone stats would be enough to be a factor.
I think placing ads at the top of the content area would be enough. I hate seeing ads within forum content. I'm sure you have all seen forums where message 2 is an advert.